The invention generally relates to permanent split capacitor single phase induction motors and, particularly, to a permanent split capacitor motor having a full capacity mode and a modulated capacity mode for improving operating efficiency.
A permanent split capacitor (PSC) motor of the type described herein has a stator assembly forming a core of magnetic material. Typically, the core consists of a stack of laminations punched from sheet-like ferro-magnetic material. Each lamination has a plurality of teeth spaced around a central opening and extending radially inwardly. When the laminations are stacked, the central openings are coaxial and constitute a bore extending longitudinally through the core. The bore receives a rotor assembly (e.g., a squirrel cage rotor) made from a stack of rotor laminations. A slip between the rotation of the rotor and the rotation of a magnetic field created by the stator induces a current in the rotor. In turn, the induced current creates a magnetic field of the rotor in contrast to the magnetic field of the stator. These contrasting rotating magnetic fields cause rotating torque of the rotor. Such a motor is particularly useful for driving a compressor of a refrigeration or air conditioning system. In this instance, the rotor has a bore for receiving a hermetic compressor crankshaft that rotatably supports the rotor body within the stator bore.
The rising cost of energy, the heightened awareness of environmental issues and the attendant governmental regulations for appliances and the like have all tended to accentuate the ongoing need for efficient and economical motors. As described above, single phase induction motors, including PSC motors, are frequently used as part of refrigeration and air conditioning systems for driving hermetically sealed compressors. In such systems, proper sizing of the equipment seeks to improve efficiency for operation over a wide range of load conditions. However, it is difficult to provide ample capacity and efficient operation for peak load conditions while still operating efficiently at lighter load conditions.
In general, the efficiency of a compressor motor involves the ratio of running load torque to breakdown torque. A ratio of about 3.0 (breakdown torque/running load torque) is desired for a relatively high efficiency for running load while still meeting the low voltage run down loaded requirements. The Air conditioning and Refrigeration Institute (ARI) sets forth standard test procedures for evaluating compressor efficiency. The test procedures examine the compressor's performance at standard conditions of 45.degree. F. evaporating and 130.degree. F. condensing temperature. Present government guidelines for energy efficiency reference ARI standards. In addition, compressor performance may be measured at operating conditions more closely approximating the actual operating conditions of a high efficiency system. For example, Copeland Corporation evaluates the performance of its compressors according to a standard referred to as "CHEER." The CHEER standard rates compressor performance at 45.degree. F. evaporating, 100.degree. F. condensing temperature; 85.degree. F. liquid; 65.degree. F. return gas. Since the CHEER rating conditions more closely approximate the conditions under which the compressor will operate most frequently, higher compressor efficiency at CHEER generally equates to lower operating cost.
One method for modulating the compressor of the refrigeration system involves operating the compressor at two distinct speeds. However, multiple speed motors often cost more than single speed motors and/or fail to provide sufficient operating torque at low speeds. As an example, distinct winding multiple speed motors require separate main and auxiliary windings for each motor speed, which can increase the cost of the motor and present problems with respect to slot fill.
Since the motor is enclosed and hermetically sealed within the compressor unit in such a system, the number of leads from the motor is another cost factor. Electrical connections are made through the shell of the compressor and special connectors are needed to preserve the hermetic seal. The use and insertion of the connectors in the shell add significantly to the cost of the compressor. Consequently, motors designed for use in hermetic compressors should incorporate a minimum number of leads so as to minimize construction problems and the cost inherent in making multiple electrical connector openings through the compressor shell.
For these reasons, a motor is desired for reducing breakdown torque and improving efficiency over a wide operating range from peak load conditions to lightly loaded conditions. Further, such a motor is desired that does not require a large number of leads.
Commonly assigned U.S. Pat. No. 4,322,665, U.S. Pat. No. 4,103,212 and U.S. Pat. No. 4,103,213, the entire disclosures of which are incorporated herein by reference, disclose single phase motors that may be used for driving compressors.